Electromagnetic instrument.



Patented Aug. 13, 1912.

APPLICATION FILED OUT. 7, 1910.

Zflnesses.

'ing at Malden, in the county of UNITED STATES PATENT OFFICE.

WALTER K. MENNS, OF MALDEN, MASSACHUSETTS, ASSIGNOB TO CHARLES H. PEARSON, OF IBROOKLINE, MASSACHUSETTS.

ELECTROMAGNETIC INSTRUMENT.

Specification of Letters Patent.

, Application filed October .7, 1910. Serial No. 585,785.

' Patented Aug. 13,1912.

To all whom'it may concern:

Be it known that I, WALTER K. Mamie, a subject of King George V of England resid- Middlesex and State of Massachusetts, have invented new and useful Improvements in Electromagnetic Instruments, of which the following is a specification.

This invention relates to improvement in indicating devices such as electricity meters, tachometers and electromagnetic speed indicators, and particularly to electro-i'na eti'c indicating devices of that class in wliic the indicating element is moved by a disk of low electrlcal resistance placed within the field of a magnetic rotor which, when rotated, sets up lines'of magnetic force. flowing through the disk producing a braking effect which tends to rotate the disk with the rotor, there being provided a spring restraining the disk and acting to return it to its initial or zero position when the ma neticrotor ceases to rotate. The 'variab e rotation of the magnetic rotor due tothe variable speed of the machine to which it is connected to be driven acts-t0 move the indicatin element to varying posit-ions correspon ing to the speed of the machine there being provided a suitable graduated scale by means of which the speed may be read. Such instruments have been found to be very seriously affected in their accurac by' variations in temperature as,for examp e, in

the case of magnetic speedometers for automobiles which are'usually calibrated at a temperature of about 68 or 70 F., actual tests have shown an inaccuracy in reading} amounting to. 163% over-registration at 163 F. and amounting to 15% under-regis-' tration at 137 .5 F., the variation per degree- This.

change'oftemperature being 27%. means a discrepancy of about ten miles per hour, and a discrepancy ofv about five miles per hour when actually traveling at thirty miles perhour. Since the reading of the instrument is proportional to the currents in- It is a act that themagpetism of the rotat-- ing' magnet decreases as the temperature which the rotor 6' rotates.

increases from normal, that is, 70 F. and increases as the temperature decreases below normal, and as the torque on the disk depends upon the square of the magnetic field 1ntensity,the variation in the strength of the magnet means double the variation in the speedometer reading. Heretofore, attempts have been made in various ways to overcome these inaccuracies, but thus far the means employed have been too expensive and delicate to be practical and have been uncertain and inaccurate in their results.

The object of my invention is to provide a simple, inexpensive-and reliable means for com ensating for changes of tem erature by c anging the relative location 0 thorotatin magnet and pivoted disk, and the relatlve location of the magnet and the field member. I

To these ends, my invention consists in the novel features of construction and in the combination and arrangement of parts set forth in the following specification-and par-- ticularly pointed out in the claims.

Referring to the drawings: Fi re 1 is a plan of an e'lectro-magnetic indicating instrument embodying my invention, the cover v being removed. Fig. 2 is a sectional view 7 the field ring, and 8 is the alumi-' num disk interposed between the -magnetic rotor and'the' field member and aflixed to a shaft 9 pivoted in bearings 10 and 11 so as to swing freely on the same axis on This disk which carries the indicating element is provided with a vertical flange 12which, in practice is rovided with suitable numerals on its periphery, not shown, which numerals are viewed through a glass window 13,

The bearings 10 and 11 and the field ring 7 are mounted in a. casing 14 which is of. non-ferrous material. When the rotor "6 is rotated it generates eddy currents in the metal disk 8, the reaction of these currents on the magnetic field ofthe rotor supplying a tor'ue to the disk so that the magnetic rotor, The "disk, however,"is restrained" pull t ereon'tends to rotate thedisk with the a suitable yielding .means. consisting of a hair spring 15 secured at its inner coil to the shaft 9 and at its outer end to a suitable fixed pin-16. This spring acts to keep the indicator steady at all speeds and return 'it to zero when the magnetic force stops.

are well known to those skilled in the art.

I will now proceed to describe the means I whereby I am enabled to compensate for .When the temperature goes below normal the changes of temperature whereby the inaccuracies of the instrument owing to variations of temperature are obviated. The casing 14 which contains the field ring 7 and the pivoted disk 8 is -supported by one or more laminated thermostats, preferably two as shown, which are designated by the numerals 17 and 18, eachjof these thermostats being preferably composed of two laminae, the upper lamina of each thermostat being formed of a materialhaving a greater coeficiency of expansion than the lower lamina. In practice, the best results have been secured with zinc for the upper lamina and steel for the lower lamina, the two strips of metal losing riveted together by" suitable rivets 19 and 20.

The thermostat 17 is supported at its left hand end upon a fixed rlng 21 forming'a part of the casing 5 and the right hand endof the thermostat 18 is supported upon said ring.':'The right hand end of the thermostat 17 and the'left end-of the ii-hell? mostat 18 are unsupported, and from these unsupported ends the casing 14, is suspended I screws 22 and 23 supported, respectively, by? the free ends of the-thermostats l'l' and 18 and screwed into the casing 14... In this way, the, casing being supported at both s1des will .movefin; a direction parallel to the vertical axis thereofL-f. 'y

The'laminse of the thermos Yd?v and 18 normal temthe instrue are so proportioned that ii perature, say 10 -.F.,: "at wh'i ment is graduated, the.'thermostatsl'gare straight and-the readings of the instrument are made correct for all." speeds, and; when the temperature varies above and belowinormal, the thermostats rise or fall, asthepase may be, at their free ends, thusraising and lowering the casing 14; together withthe ring 7 and disk 8, it being understood that "i casing is raised and when the temperature goesabove'norma1 the casing is lowered. i Since themagnetism. of the rotating magbe' iseenfthat by-pro erly proportioniiiigth'e thermostats 1'? an 18- these changes of temperature .Qanbe' compensated for by imparting to the disk 8 and the ring 7 a motion relative to the rotating magnet 6, thus-varying the gap between the rotating magnet and the disk 8 and ring 7. Consequently, although the field of the, rotating magnet varies in intensity according to changes of temperature, the torque upon the pivoted disk 8 is made uniform by the employment of the laminated thermostats and the readings of the instrument are made substantially correct for all speeds.

Having thus described my invention,

what I claim and desire by Letters Patent to secure is:

, 1. An electro-magnetic instrument hav-. ing, in combination, means for creating a rotating magnetic field including a magnet and a magnetic mass, a member arranged within the influence of said field to be rotated thereby, means for yieldingly opposing said rotation, and thermostatic means for automatically moving said; magnetic mass and said member toward and'away from said magnet to compensate for changes in temperature. v 2. An electro-magnetic instrument having, in combinaptionpmeans for creating a rotating magnetlcfield including a magnet andamagn'etic mass, a member arranged withinthe influence of said field to berotated thereby, means'for yieldingly opposing said rotation, and means for supporting saidmagnetic mass and said member including a thermostatic device whereby the relaftion of said magnetic mass and said member with respect to said magnet is automatically variedfaccording to variations in atmos- .pheric temperature, to vary the lines ,of force-efiective to rotate said member.

. 3;. electro-magnetic instrument having, 'comhination, means for creating a rotating lmagnetic' field including a magnet anus" magnetic'mass, a member arranged 'within ths influence of said field to be re-- tatedthereby, a spindle on which said memher is supported, means for yieldingly opposing the rotation of said spindle, a casmg n w chlsaid magnetic mass is supported .andl in which said spindle is pivoted, and

thermostatic means .for supporting said casing whereby the relation... of said casing, said magnetic mass and said member with respect to. said magnet isautomatically varied. according to variations in atmospheric temperature, to vary the lines of force efiective to rotate said member.

An electro-magnetic instrument having, in combination, means forcreating a rotatingma'gnetic field including a magnet and a' magnetic mass, a' member arranged within the influence of said field'to be rotated thereby, a spindle on which said memher is supported, ,me'ans-for yieldingly opposing the rotatlon of said spindle, an inner' casing in which said magnetic mass is sup orted and in which said spindle is pivote an outer casing inclosing said inner casing and said magnet, and thermostatic means for supporting said inner casing within said outer casing whereb the relation of said inner caslng, sai magnetic mass and said member with res ect to said outer casin and said magnet is automati-- cally varie according to variations in atmospheric temperature to vary the number of lines of force efiective to rotate said member.

'5. An electro-magnetic instrument having, in combination, means for creating a rotating magnetic field including a magnet and a magnetlc mass, a member arranged within the influence of said field to be rotated thereby, a spindle on which said member is su ported, means for "yieldingly opposing the rotation of said spindle, an inner casing in which-said magnetic mass is supported and in which said spindle is pivoted,

an outer casing inclosing said inner casing and said magnet, and a plurality of thermostatic devlces for supporting said inner casing at a plurality of points within said outer casing whereby the relation of said inner casing, said magnetic mass and said member with respect to said outer casin and said magnet is automatically varie according to variations in atmospheric tern-f perature to vary the number of lines of force effective to rotate said member.

I 6. An electro-magnetic instrument having, in combination, means for creating a rotating magnetic field including a magnet and a magnetic mass, a member arranged within the influence ofsaid, field to be rotated thereby, a spindle on which said member is supported,,me'ans for yieldingly opposing the rotation of said spindle, an inner casin in which said magnetic mass is sup orte and in which said spindle is pivote an outer casing 'inclosing said innercasmg and said magnet, and a pair of thermostats each composed of metal stri s having difierent heat ,coeflicients, said t ermostats being secured to said outer casing at opposite sides thereof and having their free ends extending in opposite directions, and means to secure the free ends of said thermostats to said inner casin sides thereof,-whereby the re ation 0 said inner casing, said magnetic mass and said member with respect to said outer casing and said magnet 1s automatically varied according to variations in atmospheric temerature to vary the number of lines of orce effective to rotate said member.

7. An electro-magnetic instrument having, in combination, afield member and a cooperating rotatable magnet for producing a magnetic field, a metal disk rotatably mounted in said field, and means including a laminated thermostat forming an opera tive connection between said magnet and said 'field member and disk for automatically changing the relative location of said field member and disk to said magnet to compensate for changes of temperature.

8. An electro-magneticinstrument having, in combination, a rotatable magnet, a field member, a rotatable disk interposed be tween said magnet and said field member,

a casing in which said disk is pivotall supported and-in which said field mem er is supported, and means including a laminated thermostat forming an o erative connection between said casing an said magnet for automatically chan ing the relative location of said magnet an said casing to compen-,

sate for changes in temperature.

In testimony whereof I have hereunto set my hand in presence of two subscribing witnesses.

\ WALTER K. MENNS.

Witnesses WALTER E. MOGRAW, Louis A. JONES.

at op Qsite 

